1. Field of the Invention
The present invention relates to an apparatus for detecting air bubbles in an ink jet recording head of an ink jet recording apparatus which performs recording by jetting ink droplets from a nozzle, as well as a method and apparatus, using the same detection apparatus, for recovering the ink jet recording head.
2. Description of the Related Art
Generally, a plurality of ink passages whose extreme end portions are formed into nozzles are arranged inside an ink jet recording head unit, in an on-demand type ink jet recording apparatus which produces ink droplets only when required, by changing the capacity of the ink passages by electrostrictive vibrators. At least some of the outer peripheral walls of these ink passages are formed from flexible members, and electrostrictive vibrators are provided on the outer peripheral walls. By supplying electric current to the electrostrictive vibrators at predetermined printing times, the area of the cross section of the ink passages is decreased, and ink within the ink passages is jetted from the nozzles to the paper. Thus, printing is performed.
In such an ink jet recording apparatus, a problem occurs when air bubbles are mixed into or generated in the ink passages of the recording head, filled with ink, namely, ink is not discharged. That is, if there are air bubbles in the ink passages, changes in the capacity of the passages by the electrostrictive vibrator, is absorbed by changes in the atmospheric pressure of the air bubbles. Thus, proper ink droplets cannot be jetted.
For this reason, a mechanism which has been proposed hitherto for detecting the occurrence of air bubbles inside the ink passages is disposed in the ink jet recording apparatus so that the discharge condition can be restored by removing air bubbles by a restoring apparatus when the occurrence of the air bubbles is detected.
Hitherto, various means have been proposed for detecting air bubbles in the ink passages, such as those disclosed in Japanese Utility Model Laid-Open No. 59-5570, or Japanese Patent Laid-Open No. 60-262655 and 63-141750.
However, these means require special parts to be used, parts specially constructed, or complex determination methods when these means are manufactured.
According to the above Japanese Utility Model Laid-Open No. 59-5570, a resistor is inserted into either one of the electrostrictive vibrators, and the voltage which develops at the end point of this resistor is compared with the voltage at the end point of the resistor when a capacitor and resistor are connected in series, driven in the same manner as the electrostrictive vibrators. In this method, a resistor must be inserted into each electrostrictive vibrator when a plurality of jet heads are prepared. Thus, the manufacturing method is complex.
According to the above Japanese Patent Laid-Open No. 60-262655, a voltage waveform which develops at the end point of the electrostrictive vibrator is divided by a Zener diode and a resistor so as to efficiently detect a vibration voltage. A sufficient backward electric current is required in order for the Zener diode to be in a fixed voltage range. The voltage of the electrostrictive vibrator generated by mechanical strain usually cannot supply this electric current sufficiently. The vibration voltage to be obtained attenuates, thus a sufficient amplitude cannot be supplied to the voltage comparator.
According to the above Japanese Patent Laid-Open No. 63-141750, the cycle of the vibration is measured to detect the presence of air bubbles. A counter or timer for measuring the cycle is required. This has the same drawback as Japanese Utility Model Laid-Open No. 59-5570 in that the circuit becomes quite large when a plurality of jet nozzles are driven simultaneously.